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Potentiometry titration

This document provides information on potentiometry and potentiometric titration. It discusses the basic principles of potentiometry including electrode potentials and how a potential difference is established between an electrode and solution. It describes the instrumentation used including reference electrodes like calomel and silver-silver chloride electrodes and indicator electrodes like metal, glass membrane, and quinhydrone electrodes. It also discusses different types of potentiometric titrations and provides examples of applications for potentiometry in various industries.

Education◦
Dr. B. R Ambedkar University Department of Pharmacy, Agra-282002 Guidance of Sir. Mr. Vijay Yadav
PRINCIPLE  When a metal strip is placed in a solution of its own ions there are two possibilities or tendencies:  Metal atoms may dissolve in the solution as positive ions leaving electrons on the electrode.  Metal ions may take up electrons from electrode and get deposited as neutral atoms  In this way A POTENTIAL DIFFERENCE is setup b/w electrode and solution .
POTENTIOMETER  It is instrument used to determine the potential differences between a reference electrode and an indicator electrode. These two electrodes form electro chemical cell that are dipped in solution to be analyzed.  Measured potential can be used to determine the quantity of analyte in terms of concentration.
ELECTRODE POTENTIAL Tendency of electrode to lose or gain electrons  Standard Potential Potential of pure metal when it is dipped in 1 Molar solution of its own ions at 25°C (298K) is known as standard Electrode potential.
 Oxidation Potential The potential of substance to get oxidized is called oxidation potential.  Reduction Potential The potential of substance to get reduced is called reduction potential.
INSTRUMENTATION  Potentiometric cell has following parts :  Electrodes  Salt bridge  Analytical solution  Galvanometer
WORKING At its most basic, a potentiometer consists of two electrodes, whose reduction potentials differ, inserted in a test solution. The voltmeter is attached to the electrodes to measure the potential difference between them. One of the electrodes is a reference electrode, whose electrode potential is known. The other electrode is the test electrode. The test electrode is usually either a metal immersed in a solution of its own ions, whose concentration you wish to discover, or a carbon rod electrode sitting a solution which contains the ions of interest in two different oxidation states.
1. Oxidation take place at anode. For example on zinc electrode Zn → Zn2+ + 2e- 1. Reduction take place at cathode. For example on copper electrode Cu2+ + 2e−→ Cu When redox reaction take place than potential is develop which is measured by galvanometer.
 Electrode Potential is the potential of electrochemical cell and can be represented as : E cell = E indicator - E reference +E j Ej= potential develops across the liquid junction at each end of salt bridge it is negligible .
ELECTRODES Reference Electrode 1. Standard Hydrogen electrode 2. Calomel electrode 3. Silver-Silver Chloride electrode Indicator Electrode 1. Metal Electrode 2. Hydrogen electrode 3. Quinhydrone electrode 4. Glass Membrane Electrode
REFERENCE ELCTRODES
SHE The standard H2 electrode potential is defined as the potential that is developed between the H2 gas adsorbed on the pt metal and H+ of the solution when the H2 gas at a pressure of 760 mm of Hg is in equilibrium with H+ of unit concentration Working:  Pt foil : coated with black pt  1 molar HCl solution  Pure H2 gas bubbled continuously at 1 atm.  Pt act as conductor, inert and facilitate in attaining equilibrium  Electrode Potential is 0.00 volts  H2 (gas) ↔ 2H+ (ions) + 2e- (electrons)
 Limitations : 1. Can’t be used in solution containing strong oxidizing agents. 2. Difficult and expensive to maintain. 3. Excess of H2 bubbling out carries little HCl with it and hence the H+ concentration decreases. In such a system, it is difficult to maintain the concentration of HCl at 1M.
CALOMEL ELECTRODE  Most commonly used electrode in potentiometry  Tube 5-15cm long, 0.5-1 cm in diameter.  Slurry of mercury & mercurous chloride with saturated soln of KCl  Connected by a small opening with saturated solution of KCl.  Pt metal is placed inside the slurry  Ceramic fiber act as salt bridge  2Hg +2Cl¯ → Hg2Cl2 + 2e¯  E = 0.2444 at 25°C
CALOMEL ELECTRODE
 Advantages:  Concentration of chloride ions don’t change even some of the solvent get evaporated.  Generates small junction potential so more accurate  Limitations :  Mostly saturated solution of KCl is used and that is temperature dependent.
SILVER-SILVER CHLORIDE ELECTRODE  It consist of silver wire coated with AgCl  Coating may be electroplating or Physical.  This coated wire is placed in 1M solution of AgCl.  Ag + Cl¯ ↔ AgCl + 1e¯  E= 0.199V Advantages:  Easy handling, and cost effective  Limitation: sometimes show reactivity.
SILVER-SILVER CHLORIDE ELECTRODE
INDICATOR ELECTRODES
METAL INDICATOR ELECTRODESMetal electrode develops electric potential as a result of redox reaction at its surface. 1. First Order Electrode A first order electrode involves the metal in contact with its own ions, such as Ag, Ag+ or Zn, Zn. Ag⁺ + e¯ ↔ Ag (s) E= 0.800 Limitation:  metallic indicator electrodes are not very selective and respond not only to their own cations but also to other more easily reduced cations.  – Many metal electrodes can be used only in neutral or basic solutions because they dissolve in the presence of acids
2.SECOND ORDER ELECTRODE A second-order electrode is one that responds to the presence of precipitating or complexing ions. For example, silver wire could serve as the indicator electrode for chloride. Ag +Cl¯ ↔ AgCl + e¯ E= 0.199
3. INERT ELECTRODE The most commonly used indicator electrodes are known as inert. These electrodes are not involved in the half-cell reactions of the electrochemical species. Typical inert electrodes are platinum, gold, and carbon. Inert electrodes are responsive to any reversible redox system; these are widely used in potentiometric work.
GLASS MEMBRANE ELECTRODE  Its most commonly used indicator electrode  It involves Ion Exchange reaction  Membrane is made up of chemically bonded Na2O, SiO2 and Al2O3  The Glass bulb is filled with solution of HCl & KCl, silver acetate coated with AgCl is inserted as Electrode.
WORKING  First glass absorb water  Then H ions can move in the direction of lesser concentration and replace Na ions and others in the glass membrane.  SiO2-Na⁺ + H⁺ → SiO- H⁺ + Na⁺  As a result of diffusion and exchange process a potential develops on each side of glass membrane
APPLICATIONS  Its potential is not effected by the presence of strong reducing and oxidizing agents.  It operates over a wide pH range,  It respond fast and function well in physiological systems  Used in pH meter.
HYDROGEN ELECTRODE  Used in acid base titration but very limited application because many organic substances directly react with hydrogen gas.
QUINHYDRONE ELECTRODE  Contain a solution of Quinones and Hydroquinones prepared from Quinhydrone  2QH → Q+H2Q  Simple and easily attains equilibrium  employed in the presence of mild oxi and red agents at pH8  Sensitive to high conc of salts and oxi & red agents.
 The electrode consists of an inert metal electrode (usually a platinum wire) in contact with quinhydrone crystals and a water-based solution. Quinhydrone is slightly soluble in water, dissolving to form a mixture of two substances, quinone and hydroquinone, with the two substances present at equal concentration. Each one of the two substances can easily be oxidised or reduced to the other.  The potential at the inert electrode depends on the ratio of the activity of two substances (quinone-hydroquinone), and also the hydrogen ion concentration. The electrode half- reaction is:  Hydroquinone ↔ Quinone + 2H+ +2e-
 For practical pH measurement, a second pH independent reference electrode (such as a silver chloride electrode) is also used. This reference electrode does not respond to the pH. The difference between the potential of the two electrodes depends (primarily) on the activity of H+ in the solution. It is this potential difference which is measured and converted to a pH value.  The quinhydrone electrode is not reliable above pH 8. It is also unreliable in the presence of strong oxidising or reducing agents, which would disturb the equilibrium between hydroquinone and quinone. It is also subject to errors in solutions containing proteins or high concentrations of salts.  Other electrodes commonly used for measuring pH are the glass electrode, the hydrogen electrode, the antimony – antimony oxide electrode, and the ion-sensitive field effect transistor ISFET electrode.
SALT BRIDGE  U shaped tube filled with an inert electrolyte 1. Glass tube bridge (gel+ KI or Na2SO4) 2. Filter tube bridge (filter paper+KCl or NaCl) Function: 1. Allow electrical contact b/w 2 solutions. 2. Prevent mixing of two solutions. 3. Maintain electrical neutrality.
TITRATION :  Quantitative measuring procedure in which a liquid solution is added to a mixture until some distinctive feature, signal or end point is observed.
POTENTIOMETRIC TITRATION  It’s a volumetric method in which potential between two electrodes (reference & indicator) is measured as a function of added reagent volume.
TYPES 1. Precipitation titration 2. Complex formation Titration 3. Neutralization titration 4. Oxidation – reduction titration
PRECIPITATION TITRATION  Volumetric methods based upon the formation of slightly soluble precipitate are called " precipitation titration " .  Because of the precipitating titration based upon utilizing silver nitrate  (AgNO3) as a precipitating agent, then it called " argentimetric  processes " .  Precipitation titration is a very important , because it is a perfect method  for determine halogens and some metal ions
COMPLEX FORMATION TITRATION  Complex ions ( coordination compounds) are produce from reaction of  many metal ions (electrons accepter) with electron pair donors .  The donor species (or called ligands) must have at least one pair of  unshared electrons for bond formation
NEUTRALIZATION TITRATION  Neutralization titrations are performed with standard solutions of strong acids or  bases. While a single solution (of either acid or base) is sufficient for the titration of  a given type of analyte, it is convenient to have standard solutions of both acid and  base available in case back-titration is needed to locate the end point more
OXIDATION-REDUCTION TITRATION  A redox titration is a type of titration based on a redox reaction between the analyte and titrant. Redox titration may involve the use of a redox indicator and/or a potentiometer. Common examples of a redox titration is treating a solution of iodine with a reducing agent and using starch as an indicator. Iodine forms an intensely blue complex with starch. Iodine (I2) can be reduced to iodide(I−) by e.g. thiosulphate (S2O3 2−), and when all iodine is spent the blue colour disappears. This is called an iodometric titration
APPLICATIONS OF POTENTIOMETRY  Analysis of pollutants in water  Drug Analysis in Pharmaceutical industry  Food industry for analysis of quality  Biochemical and biological Assay or analysis To check the quality of cosmetics  Also used as analytical tool in Textile, paper, paints, explosive energy and more .
Potentiometry titration
Potentiometry titration